storage: Decode keys of storage entry iterations

Fetching a query of all systems' accounts storage().system().account_iter() returns a key value struct:

• keys contain ()
• key_bytes contains twox System twox AccountIter [remaining-key-of-the-account]
• value contains the account value

https://github.com/paritytech/subxt/blob/b076f4c66ca2b7627f32d899211ea99a47db1c28/subxt/src/storage/storage_type.rs#L313-L323

From the user's perspective, the keys field makes it a bit harder to reason about the final key we provide.
In this example, keys field does not return the account ID part of the key.

In codegen, we pass () as keys to the account_iter fn. However, we know that the keys field in this case is similar to the account fn:

:subxt::ext::subxt_core::storage::address::StaticStorageKey::new(
   ::subxt::ext::subxt_core::utils::AccountId32(pub [u8; 32])
)

Example to reproduce this:

#![allow(missing_docs)]

use std::str::FromStr;
use subxt::ext::codec::Decode;
use subxt::utils::{AccountId32, H256};
use subxt::{Config, OnlineClient, SubstrateConfig};

type ChainConfig = SubstrateConfig;

#[subxt::subxt(runtime_metadata_insecure_url = "wss://rococo-asset-hub-rpc.polkadot.io")]
pub mod asset_hub {}

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let client =
        OnlineClient::<ChainConfig>::from_url("wss://rococo-asset-hub-rpc.polkadot.io").await?;

    let storage = client.storage().at_latest().await.unwrap();

    // Build a storage query to iterate over account information.
    let storage_query = asset_hub::storage().system().account_iter();

    // Get back an iterator of results (here, we are fetching 10 items at
    // a time from the node, but we always iterate over one at a time).
    let mut results = storage.iter(storage_query.clone()).await?;
    while let Some(Ok(kv)) = results.next().await {
        println!("Decoded key(s): {:?}", kv.keys);
        println!("Decoded key_bytes: {:?}", hex::encode(kv.key_bytes));
        println!("Value: {:?}", kv.value);
    }

    Ok(())
}

Start with subxt/src/storage/storage_type.rs at lines 313-323 and the generated storage().system().account_iter() entry point shown in the issue. Run the provided Rococo Asset Hub reproduction and inspect how the codegen currently passes () as the key type. Done means iteration keys expose the account ID rather than an empty tuple, while key_bytes and the account value remain available.